後2世紀におけるローマ帝国の東方への拡大は, 従来, トラヤヌス帝以来の3度のパルティア戦争を中心に考察されてきた。本稿は, その狭間で, 防衛に徹し対外平和を維持したとして軽視されてきたハドリアヌス帝とアントニヌス・ピウス帝の治世に着目した。帝国領自体は拡大しなかったが, この時代にこそローマ帝国は東方諸国に対する統制を強めていき, 対外的権威が著しく増大したのである。とりわけ, 黒海からアルメニアにかけての地域への支配はアントニヌス・ピウス帝の下で確立し, マルクス・アウレリウス帝のパルティア戦争はこの延長線上で理解すべきものであった。さらに, この体制を基盤として, セプティミウス・セウェルス帝以後, ローマがメソポタミアへ直接支配を拡大していくこともまた可能となったのである。
The Emperor Hadrian withdrew his army from the area beyond the Euphrates and stopped governing Armenia, Mesopotamia, and Assyria as Roman provinces. This may appear to have been against the policy of the former Emperor Trajan, who had originally created the provinces after the Parthian War. However, it is worth noting that Trajan himself had already converted some areas in the provinces into dependent kingdoms during his reign in order to allay the rebellious mood of the indigenous peoples. Hadrian succeeded and completed this process of changing the direct rule over them into an indirect one. Rome thus successfully retained its influence over most kingdoms beyond the Euphrates. However, this caused conflict between Rome and Iberia, a kingdom in the centre of the Caucasus. During Hadrian's reign, Rome extended its power over Colchis and Armenia, east and south of Iberia. This prevented Iberia from expanding its territory, obliging it to cooperate with the Alani, who lived in the northern Caucasus. The situation of the northeast frontier was an unstable element in the Roman control of the area beyond the Euphrates. When Antoninus Pius acceded to the throne, the situation changed. He summoned Pharasmanes, the king of Iberia, to Rome and forced him to acknowledge Roman sovereignty. Rome thus gained firm control of the Caucasus, the hinterland of Armenia, and the stabilization of its influence over the area from the Black Sea coast to Armenia led to the creation of Rome's great advantage over Parthia. This is why Marcus Aurelius was able to deal with the Parthian invasion and strike back easily, and why Septimius Severus was able to create provinces in Mesopotamia. The period from Hadrian to Antoninus Pius, in which the Roman Empire is generally believed to have been peaceful and defensive, must have been crucial for the establishment of the hegemony of Rome in the East.